This invention relates to high pressure discharge devices and particularly to an improved envelope configuration for devices such as mercury arc lamps and to a method for controlling the lamp impedance and increasing the lamp life. Such lamps have particular application in the manufacture of television picture tubes and in integrated circuit fabrication.
Commercially available mercury arc lamps comprise a sealed fused quartz tubular envelope having lead-in conductors sealed through and extending a short distance into the envelope and terminating in electrodes of the same material as the lead-in conductors. The lamps further comprise pools of mercury contained within the envelope and positioned in chambers at the ends of the envelope surrounding the electrodes. Such lamps exhibit a limited useful life of about 100 hours because of devitrification of the fused quartz envelope and because of an increase in lamp impedance. Devitrification means a change in the glass-like properties of the envelope material, particularly the transparency.
A solution to the devitrification problem was addressed in copending application Ser. No. 901,821, filed May 1, 1978 by Carl G. Hernqvist and entitled "Improved Mercury Arc Lamps." That application discloses that devitrification can be controlled and lamp life extended to several hundred hours by using fused silica rather than fused quartz as the envelope material, wherein the fused silica has a total metallic impurity of less than one part per million by weight and an OH radical content of less than 5 parts per million by weight. The charge in envelope material, however, did not eliminate the problem of increased lamp impedance during operation.
The problem of increased lamp impedance is addressed by the present invention. The cause of increased lamp impedance is not precisely known, but it is thought to be due to the progressive increase in vapor pressure within the lamp. As the distance from the electrode tips to the face of the mercury pools decreases, more mercury is vaporized causing an increase in vapor pressure and an increase in impedance. The distance change could be the result of electrode erosion either from extended lamp life in those lamps using the fused silica envelope, or in conventional lamps using the fused quartz envelope, from higher lamp power required to maintain a constant lamp brightness as the transmission of the fused quartz decreases due to devitrification. Also, the inability to accurately fill the lamps with mercury causes some lamps which are over-filled to initially have a relatively high impedance which continues to increase during operation.